Many pathologies manifest clinically through unwanted or excessive immune responses within a host, e.g., transplant rejection, inflammatory and autoimmune disorders. Immunosuppressive therapies have been developed to treat the symptoms, but not the underlying cause of pathologies characterized by excessive immune responses. These therapies are effective at down-modulating immune function and, as such, carry the potential for severe adverse events, including cancer and opportunistic infection, as well as side effects such as cataracts, hyperglycemia, bruising, and nephrotoxicity from agents such as prednisone, cyclosporine, and tacrolimus.
Although therapies that do not suppress the entire immune system have been developed, there are limitations associated with these regimens as well. These immunomodulatory treatments target a narrower point of intervention within the immune system and, as such, have different, sometimes less severe side effects. Examples of such immunomodulatory therapies include the use of antibodies, e.g., anti-CD3 or anti-IL2R. While successful at inducing a heightened state of non-responsiveness, the withdrawal of these immunomodulatory therapies results in a reversion to the unwanted pathology.
Mesenchymal stem cells (MSC or MSCs) are multipotent stem cells with self-renewal capacity and the ability to differentiate into osteoblasts, chondrocytes, and adipocytes, among other mesenchymal cell lineages. In recent years, the intense research on the multilineage differentiation potential and immunomodulatory properties of human MSC has indicated that these cells can be used to treat a range of clinical conditions, including immunological disorders as well as degenerative diseases. Consequently, the number of clinical studies with MSC has been steadily increasing for a wide variety of conditions: graft-versus-host disease (GVHD), myocardial infarction and inflammatory and autoimmune diseases and disorders, among others. Currently, clinical programs utilizing MSCs rely on isolation of these cells from adult sources and cord blood. The high cell doses required for MSC clinical applications (up to several million cells per kg of the patient) demands a reliable, reproducible and efficient expansion protocol, capable of generating a large number of cells from those isolated from the donor source.
To reach the clinically meaningful cell numbers for cellular therapy and tissue engineering applications, MSC ex-vivo expansion is mandatory. Sequential ex-vivo cell passaging of MSCs from cord blood, fetal and adult sources (such as bone marrow or adipose tissues) can cause replicative stress, chromosomal abnormalities, or other stochastic cellular defects, resulting in the progressive loss of the proliferative, clonogenic and differentiation potential of the expanded MSCs, which ultimately can jeopardize MSC clinical safety and efficacy. The issues with use of senescent MSCs in treatment should not be underestimated since cells lose part of their differentiation potential and their secretory profile is also altered. MSC senescence during culture was found to induce cell growth arrest, concurrently with telomere shortening. A continuous decrease in adipogenic differentiation potential was reported for bone marrow (BM) MSC along increasing passages, whereas the propensity for differentiation into the osteogenic lineage increased.
Accordingly, some essential problems remain to be solved before the clinical application of MSC. MSCs derived from ESCs can be generated in sufficient quantities and in a highly controllable manner, thus alleviating the problems with donor-dependent sources. Since long-term engraftment of MSCs is not required, there is basically no concern for mismatch of major histocompatibility (MHC) [7,8]. In the art, MSCs derived from ESCs have been obtained through various methods including co-culture with murine OP9 cells or handpicking procedures [9-13]. These methods, however, are tedious and generate MSCs with a low yield, varying quality and a lack of potency. Moreover, maximizing the potency of the injected cells is desirable, in terms of being able to provide a cellular product with a better therapeutic index, ability to be used at a reduce dosage (number of cells) relative to CB-derived, BM-derived or adipose-derived MSCs, and/or the ability for the MSCs to provide a tractable therapy for inflammatory and autoimmune diseases for which CB-derived, BM-derived or adipose-derived MSCs are not efficacious enough.